Although the present invention is hereafter described with respect to video equalizing amplifiers for compensating losses associated with coaxial cable used by the professional television industry, it is to be understood that the invention can be applied to any situation involving the transmission of any type of signal over any type of frequency dependent network where it is desired to restore a corrected (typically flat) overall frequency response.
Analogue TV signals are distributed in a system via coaxial cables. Such cables have a signal loss characteristic which is frequency-dependent, with the loss at the higher video frequencies (e.g. 5 MHz) being much greater than the loss at low frequencies (e.g. below 100 KHz). When the length of such cables exceeds 30 feet, the effect of relatively greater attenuation of the higher frequencies may become objectionable because of the loss of picture detail and color saturation. The loss of color occurs because the NTSC color system employs the encoding of color information onto a 3.58 MHz subcarrier (4.43 MHz in the PAL system) and saturation is in proportion to modulation amplitude.
It is common practice to employ equalizing amplifiers to compensate for such cable losses. The adjustment of such prior-art amplifiers to match a particular cable length is a complex procedure. It normally involves several adjustments, each of which apply primarily to a different part of the frequency spectrum but which typically interact. To accomplish precise equalization requires the availability and use of frequency sweep generators and associated measuring equipment. This procedure is further complicated by the distance between the ends of the cable, which represent the generator and measuring equipment locations. In general, the procedure is quite difficult and only the largest and best installations have the necessary equipment to do this properly.
An amplifier which can equalize a given fixed length of cable is shown in FIG. 1. In this example, the equalizing network 2 provides high frequency slope adjustment and comprises a complex multi-section design with a separate adjustment of the higher frequencies and perhaps midfrequency adjustments as well. The amplifier 5 has a flat response and adjustable gain (the gain may be set to zero). The resultant correction signal, which is obtained by appropriately setting the gain of amplifier 3, is added to the incoming signal via summing circuit 4 to provide an output with corrected (flat) frequency response.
At first, it may seem that, once the equalizing network 2 is calibrated, various lengths of cable may be accommodated by appropriately setting the variable gain amplifier 3, but this is not so. Consider the case where amplifier 3 and network 2 have been calibrated for 500 feet of cable. If the cable length is increased to 1000 feet, it might seem that if the gain of amplifier 3 were doubled, the cable compensation would be correct. In fact, the high-frequency compensation would not be sufficient.
If, for example, for 500 feet of cable, the amount of 100 KHz signal passed is 95% and the amount of 10 MHz signal is 70%, then it follows that for the 1000 feet cable, the output would at 100 KHz be 90.25% (0.95.times.0.95) and at 10 MHz would be 49% (0.7.times.0.7). Thus, as the cable length is increased, the input to the filter network 3 becomes deficient in high frequencies; therefore, the filter shape must change in order to meet the larger demand for high frequency correction. Obviously, a constant filter response will not work for both cases. There is a cascading effect which requires a different equalizing network for the longer cable. This is the reason for the multiple adjustments in the prior-art amplifiers.